So it looks like ya got a couple of issues here. As the peeps mentioned, acid - weather in vinegar (preferably stonger like 30%+ if available) or chemicals to take off the lime deposits is a first start. Second it looks like the top layer(s) of the enamel are damaged and or cracked which is why you are struggling with it "coming back and not completely going away". There are fixes for this, but replacement is usually recommended if this is the case.